When the Poles Flip
More than 20 years ago, the first conclusive evidence was found on the sea floors that the earth's north and south poles have switched places repeatedly over the course of geologic time. The last reversal occurred some 710,000 years ago. For most of a million years before that, the north pole was the south pole and vice versa. Epochs of shorter duration on the order of 50,000 years can be found in the geologic record, but for the most part, each seems to last between 200,000 and 1,000,000 years.
No one knows how long it takes to make the transition. It may take a few years or it may take a few thousand (a few thousand years is about the shortest time interval that can be reliably measured from sediments and lava flows on the ocean floor). What is known from measurements of remnant magnetism in ancient pottery shards is that the earth's magnetic field has weakened by more than 50 percent in the past 4,000 years. In other words, we may be headed into another reversal.
What would happen to us during a period of reversal of the earth's magnetic field? The people who were around during the last one were not much for keeping records, and there's no way to tell.
Obviously, every guidance system on earth that uses a compass would be affected, but this would not be a serious problem for a civilization that rarely relies on them any more. But it might be a different matter for the birds, fish, and other creatures (down to and including some bacteria) that rely on the earth's magnetic field for their orientation or for guidance on migratory routes. The very fact that they have survived through other flip-flops indicates that such changes must take at least long enough to give successive generations time to adapt.
A more unsettling conjecture is that earlier pole reversals may have been responsible for mass extinctions on earth. Changes in the magnetic field do not affect most living organisms directly (you could stand in the strongest magnetic field ever made and not feel a thing), but during the time that the field passes through "neutral" on its way from one polarity to the other, the earth will temporarily lose much of its protection from cosmic radiation.
In a normal environment, incoming cosmic rays and particles in the solar wind are deflected poleward by the magnetic field. When the field is reduced to a very low level during a reversal, a large percentage of these rays and particles would be able to reach the surface of the earth. This increased radiation could conceivably result in mutations or extinctions.
Weather patterns would almost certainly be affected, damaging food supplies or placing species under other strains to which they might not be able to adapt. Certainly such difficulties wouldn't help, but a myriad of reasons have been suggested for ancient extinctions, and this is only one of them.
Although there is no doubt that magnetic field reversals have occurred in the past, the reasons for this aberrant behavior remain obscure. The most likely cause is that secular variations (short-term variations on the order of a few hundred years) in the magnetic field may trigger a complete reversal of the stronger dipole or long-term, field that defines which way is magnetic north and which is south.
The dipole field regularly undergoes cycles of strong and weak intensity, each cycle lasting about 10,000 years. Statistically, there is evidence that about every 20 to 25 cycles, the strength of the dipole field is at its weakest at the same time that the secular is at its strongest, but in the opposite direction. This may be enough to flip the main field on its back, much like a player at Indian wrestling taking advantage of a stronger opponent at an opportune moment.
